When different types of semiconductors are connected to form an electric circuit and direct current is applied, heat is produced at one junction and heat is absorbed at the other junction. This phenomenon is called the Peltier effect. Electronic cooling of a target using the Peltier effect is called thermoelectric cooling, and devices that provide such cooling are referred to as thermoelectric coolers or Peltier coolers. Further, if there is a temperature difference between two junctions, an electromotive force is generated in proportion to the temperature difference. This phenomenon is called the Seebeck effect, and power generation using such generated electromotive force is called thermoelectric power generation. In addition, a temperature sensor called a thermocouple senses a temperature difference between two junctions based on a junction between two metals that is included in an electric circuit. Measuring the thermoelectromotive force between two junctions permits temperature determination. In addition to the thermocouple, a variety of sensors based on the Seebeck effect include a device, a module, or a system for obtaining a change in a quantity of intensive property (intensity variable) by sensing the temperature difference by a potential difference, the change having one-to-one correspondence with a temperature difference, and for giving feedback to a variety of functions. Such an element having a basic configuration for connecting a different type of metal or a semiconductor is generally called a thermoelectric element, and metal or a semiconductor used for the element with a high thermoelectric performance is called a thermoelectric material.
Since thermoelectric cooling is a cooling by a solid element, it is characterized in that no toxic refrigerant gas is necessary, no noise occurs, and partial cooling is possible. In addition, because heating is possible in a Peltier effect device by switching a direction of current, temperature can be regulated with accuracy. Typical applications include cooling and precise temperature regulation of electronic components and temperature control of storage cabinets such as wine coolers in which temperature control is important. When a thermoelectric material with high performance is used at room temperature or below, it is possible to achieve a refrigerator, a freezer and a seat cooler without using any toxic gas such as a CFC. Meanwhile, thermoelectric power generation realizes effective use of energy that includes power generation using waste heat from a heat engine such as a factory, a power plant and a vehicle, power generation using abundant solar energy, a wearable device such as a thermoelectric power generation watch using a temperature difference between a body and outside air, or the like. Further, a thermoelectric material with large thermoelectromotive force and a small resistance has a high usage value as a temperature sensor such as a thermocouple having a high sensitivity.
Although a bulk material is often used for the above applications, application of a thin film type Peltier element is expected for the cooling and temperature regulation of small parts. For example, a CPU or the like has a problem of performance reduction in computing speed and the like due to a temperature increase of a semiconductor element during operation associated with higher speed and performance and a smaller size and thickness of the element. A thin film type of a thermoelectric cooling element with a thickness of 500 μm or less and high performance is required for cooling such a part. An important problem particularly in this application is to transport heat generated in components to the outside as quickly as possible, thereby maintaining the temperature of components to prevent a temperature increase. Therefore, performance such as a power factor to be described below may be regarded as important in a thin film type material other than a figure of merit demanded of a bulk material.
High performance of a thermoelectric element is generally indicated by a fact that any one of thermoelectromotive force (V), a Seebeck coefficient (α), a Peltier coefficient (π), a Thomson coefficient (τ), a Nernst coefficient (Q), an Ettingshausen coefficient (P), electrical conductivity (σ), a power factor (PF), a figure of merit (Z), and a dimensionless figure of merit (ZT) is high, or any one of thermal conductivity (κ), a Lorentz number (L), and electrical resistivity (ρ) is low.
Particularly, a dimensionless figure of merit (ZT) is indicated by ZT=α2σT/κ (here, T indicates an absolute temperature) and is an important element for determining efficiency of thermoelectric conversion energy such as a coefficient of performance in thermoelectric cooling and conversion efficiency in thermoelectric power generation. Therefore, it is possible to increase efficiency of cooling and power generation by using a thermoelectric material with a large figure of merit (Z=α2σ/κ) to form a thermoelectric element.
Namely, a thermoelectric material preferably has a large Seebeck coefficient (α), a large electrical conductivity (σ), therefore a large power factor (PF=α2σ) and a small thermal conductivity (κ). Moreover, in other words, a material preferably has a large Seebeck coefficient (α) and a large ratio σ/κ(=1/TL) of an electrical conductivity to a thermal conductivity.
In the application for thermoelectric power generation, a material having a large power factor as well as a large figure of merit may be required. A performance index (Z) is a value obtained by dividing a power factor (PF=α2σ) by a thermal conductivity (κ). When κ is small, a figure of merit is increased as for the same power factor. However, when κ is too small, since an element is inserted in a part having a temperature difference, a thermal resistance rises, resulting in a large system and increased capital and operating costs. See, for example, Yamaguchi et al., Thermoelectric Conversion Symposium, Tokyo, Aug. 6, 1999, Tokyo, at 44.
As a material used for a variety of sensors, an absolute value of the Seebeck coefficient of at least 50 μV/K at room temperature is needed for increased measurement sensitivity and precision. A metallic thermoelectric material such as copper-constantan (α is about −50 μV/K at room temperature), Alumel-Chromel, and platinum platinum-rhodium is usually used. However, since these are made of precious metals or use multicomponent alloys, the material and manufacturing costs are high.
Most of the materials conventionally used as thermoelectric elements are multicomponent semiconductor materials doped with various materials in a crystal structure of a Bi2Te3 system. On the other hand, some of the materials, called strongly corrected 4f electron materials that contain rare earth elements such as YbAl3, CePd3 and CeRhAs, show high thermoelectric properties in a temperature region lower than room temperature. Although they are useful at low temperatures (10 to 200 K), good properties have not been achieved around room temperature. There have been efforts to prepare a material with high performance at room temperature or below by adding a rare earth element such as Ce, Sm or Yb which is an essential component in the 4f electron material to a Bi2Te3 crystal. Although properties of such a material may attract interest, such a material is not sufficiently investigated. P. G. Rustamov et. al., Physica Status Solidi A: Applied Research (1984) 86 (2), K113-K115, describes a CeBiTe3 (Ce20Bi20Te60) material having a hexagonal crystal structure, but having poor oxidation resistance since a large amount of rare earth elements is added. Furthermore, although the observed value of electrical conductivity is unknown, this material is said to show a semiconductor-type change of electrical conductivity with temperature. Therefore, based on the change of the Seebeck coefficient with temperature, it can be expected that this material is suitable for high temperature applications at 400 K or higher.
An example of preparing a Ce2Te3—Bi2Te3 solid solution in a region where the solid solubility of Ce2Te3 is low has been reported in P. G. Rustamov et. al., Zhurnal Neorganicheskoi Khimii, (1979), 24(3), 764-766), although this material is not regarded as a thermoelectric material. According to this report, thermodynamically stable hexagonal CeBiTe3 is a stoichiometric intermetallic compound, and it is unlikely that a non-stoichiometric solid solution can be formed with a similar composition. In addition, from a phase diagram reported in the above paper, it can be understood that Ce2Te3 has an upper limit of the solid solubility of 3 mol % based on Bi2Te3 at room temperature in a thermodynamically stable phase. However, there is no report in the foregoing literature on a material which has a non-stoichiometric ratio of the sum of the content of Bi and Ce to the content of Te.
An amorphous or slightly crystalline R—Bi—Te material is disclosed in Japanese Patent Laid-Open No. 08-111546. Since this material is not a highly crystalline material, it has poor stability to thermocycling, variation with time and the like. In addition, the report describes that it has a low electrical conductivity. Accordingly, this material will have a poor power factor and thus will have disadvantageous features for applications such as thin film type thermoelectric materials and thermoelectric power generation.
A thermoelectric power generation element generates power using a temperature difference between a higher temperature side and a lower temperature side, and a thermoelectric cooling element generally functions by the heat transferred by electric current from a lower temperature side to a higher temperature side. Therefore, these elements are each inserted into a part with different temperatures. Accordingly, a difference in thermal expansion appears between a lower temperature side and a higher temperature side, creating a thermal shear-stress in an element. Occurrence of the shear stress reduces the life of a thermoelectric semiconductor element by thermocycling. For some applications, a thermoelectric semiconductor material composed of a melted polycrystal or a sintered-compact powder has been used which has a relatively high shear stress without cleavability, and various connections and element structures have been proposed. However, these may have insufficient alignment compared with a single crystal, reducing thermoelectric performance to be expected, and special connection methods may limit usage thereof. A thermoelectric material which can satisfactorily endure the above described thermal shear-stress has not yet been found.
An object of the present invention is to provide a thermoelectric material which can be expected to have high performance as a thermoelectric element, that is, which has a high Seebeck coefficient (α) and a high a power factor (PF), when used in a temperature region of from −50° C. to 100° C. Another object of the present invention is to provide a thermoelectric material for thermoelectric power generation or a thermoelectric element for thermoelectric cooling which has solved the above described various problems caused by brittleness of a thermoelectric semiconductor and has a satisfactory figure of merit (ZT).